1. Field of the Invention
The present invention relates to an expandable bladder for tyre-curing apparatuses made up of two or more elastomer materials having different compositions, disposed in several mutually-coupled layers, for example.
It is also an object of the invention to provide a method of manufacturing said bladder.
2. Description of the Related Art
In the embodiments that will be disclosed in the present description, the invention is conceived to be utilised within the scope of tyre manufacturing processes, and more particularly in the moulding and vulcanisation steps of previously built green tyres.
It is known that in the manufacture of tyres for vehicle wheels a moulding and vulcanisation treatment is provided subsequently to a step involving building of the green tyre by assembling of the respective elastomer material components with appropriate reinforcing structures, said moulding and vulcanisation treatment aiming at determining the structural stabilisation of the tyre by cross-linking of its elastomer components and, if required, imparting a desired tread pattern thereto as well as possible graphic marks at the sidewalls.
To this aim, the green tyre is introduced into a suitably heated vulcanisation mould having a moulding cavity conforming in shape to the final conformation to be given to the tyre itself. After carrying out closing of the mould, a bladder of toroidal shape is allowed to expand within the tyre, which bladder in a radially internal position has at least one circumferential edge carrying tailpieces for anchoring to respective parts of the mould. Bladder expansion is obtained by admitting steam under pressure thereinto, so as to bring the bladder into contact with the inner tyre surface and compress the tyre against the moulding cavity walls, with simultaneous heat transmission in an amount required for cross-linking.
When vulcanisation has been completed, the mould is opened to allow removal of the tyre and set the mould ready for a new vulcanisation cycle.
However, it is necessary to prevent the bladder from adhering to the inner walls of the tyre too much, to promote both adaptation of the bladder to the tyre conformation and tyre removal without any risk of damage at the end of vulcanisation.
As known, this requirement is tackled by resorting to surface treatments essentially consisting in disposing appropriate release agents usually applied by spraying, on the outer bladder surfaces and/or the inner tyre surfaces.
These treatments however involve different problems originating from the material employed as the release agent and the equipment and time required for carrying out the treatment itself. Surface treatments can also create problems in terms of environmental impact, because the materials used as the release agents can be dangerous, as well as in terms of production rejects that may occur each time the surface treatment is not uniformly carried out over the whole bladder/tyre contact surface.
The state of the art proposes different solutions to reduce adhesion of the vulcanisation bladder to the tyre surfaces. For instance, document JP 63125311 discloses how to manufacture an expandable bladder with an elastomer composition comprising an organic rubber and a polyorganosiloxane containing a specific silicone-modified elastomer as the solubilizing agent.
In document JP 5031724 use of an expandable layered bladder is proposed which has an inner elastomer layer consisting of butyl rubber and an outer elastomer layer consisting of silicone rubber. The inner layer of butyl rubber ensures a hermetic seal to steam under pressure, whereas the outer layer of silicone rubber facilitates sliding of the bladder on the inner tyre surfaces and release therefrom when vulcanisation is over.